Ambient bioaerosols in PM10 samples were measured at three sites in Corcoran, an agricultural town in the southern San Joaquin Valley (SJV) of California, during fall of 2000 corresponding to the cotton harvest season. Elevated bioaerosol concentrations were measured near grain elevators (GRA site) and a cotton handling facility (BAI site) as compared to levels in a residential community (COP site), ~2 km northeast of these sources. Average endotoxin levels (13 ± 17 EU/m3) at the grain elevator site were three to eight times higher than averages at the nearby cotton-handling and residential sites. The highest level (47.6 EU/m3) at the grain elevator site was about half of the exposure limit of 90 EU/m3 set by the Dutch Expert Committee on Occupational Safety. Particle counts of fungal spore (66,333 particles/m3) and pollen grain (2,600 particles/m3) concentrations were more than double those reported in the literature. Average fungal biomarker concentrations of 170 and 131 ng/m3 for arabitol and mannitol, respectively, were 1–2 orders of magnitude higher than those from nonagricultural areas. The low correlation (r < 0.11) of three fungal markers (i.e., (1→3)-β-D-glucan, arabitol, and mannitol) with fungi counts is consistent with findings by others and indicates that these are insufficient as surrogates to represent fungal exposure. Agricultural activities contributed measureable amounts to PM10 mass and organic carbon (OC), dominated by fungal spores (i.e., 5.4–5.8% PM10 mass and 11.5–14.7% OC). The sum of fungal spores, pollen grains, and plant detritus accounted for an average of 11–15% PM10 and 24–33% OC mass. Bioaerosols can be important contributors to PM10 mass in farming communities similar to Corcoran, especially during intense agricultural activities.